I've dried blueberries before and it takes forever to get them rock hard..More than 24 hrs.
I dried some frozen ones this past week , they were for someone else.. She wanted them at the raisin stage.
After about 24 hrs.. some where like a rock and some were still mush.
I couldn't afford the time to run the dehydrator for another 24 hrs with a limited volume of berries.
I picked neutral squash to not affect the flavor of the berries and continued running the machine.
The moisture added back in, plumped the berries to uniform raisin consistency...No rocks and no mush.
Adding a controlled amount of moisture back in may be the key.
Interesting obsevations, and I think you are correct.
I have never dried to dry blueberries.
Good for you, I am glad you are dehydrating and sharing your knowledge here.